1. Field of the Invention
The invention is in the field of integrated optical devices. More in particular, it relates to integrated optical devices for manipulating the polarization of optical signals, in this case splitting, combining or filtering.
2. Prior Art
integrated optical devices which deal with polarization, such as polarization splitters, are used, inter alia, in integrated embodiments of coherent optical receivers, in which detection takes place on the basis of polarization diversity. In a global sense, two kinds of integrated optical polarization splitters of this type are known. The one kind is based on interference, use being made, for the purpose of splitting two mutually orthogonal polarizations TE and TM of/in an optical signal, of the difference in phase between a fundamental and a first-order propagation mode of the polarizations. The other kind is based on the so-called "mode sorting effect". In this case, use is made of the difference in propagation preference of the polarizations for the two output channels, which differ from each other in terms of propagation constant, of an asymmetric Y-junction. The propagation preference is based on the fact that the asymmetry for the two polarizations differs in sign, which can be achieved by using birefringent materials. Thus, reference [1] discloses a polarization splitter on lithium niobate, with a waveguide structure, obtained by means of Ti diffusion therein, for the Y-junction, the opposite asymmetry being based on anisotropy in the increase of the refractive index as a result of the Ti diffusion. Reference [2] discloses a polarization splitter with a waveguide structure for the Y-junction on the basis of transparent polable polymer. In this case use is made of the fact that polable unpoled polymer is not birefringent, whereas poled polymer is, the poled state showing, with respect to the unpoled state, a refractive index difference which, for the two polarizations, differs in sign between one another.
Since at present it is customary, in optical communication systems, to choose a wavelength for the optical signal in the near infrared, an integrated optical receiver provided with a polarization splitter of this type can, given the current prior art, only be provided on the basis of semiconductor material such as indium phosphide (InP). A polarization splitter of the first type, implemented on InP, is disclosed, for example, by reference [3]. This known polarization splitter makes use of the polarization-dependent effect of a metal layer on the propagation of the guided modes in a directional coupling structure. Such a directional coupling structure provided with a metal layer, however, makes stringent demands on fabrication technology and presents additional complications during manufacture thereof. In a polarization splitter based thereon, the presence of the metal layer leads to unwanted additional attenuations of optical signals propagating therein. In a patent application not published at the time of filing, to wit reference [4], a polarization splitter is described which can be implemented very readily on InP and does not have the drawbacks mentioned of the polarization splitter disclosed by reference [3]. This known polarization splitter, which can be regarded as a kind of hybrid of the two kinds of polarization splitters indicated, consists of a mode converter having a periodic structure, in which one of the two polarizations is converted into a different order of guided mode, in combination with an asymmetrical Y-junction. Both the known polarization splitters which can be implemented on InP have the drawback, however, that they show a strongly wavelength-dependent behaviour.
The splitters of the above-indicated second kind have major advantages compared to those of the first kind. Namely, they are less wavelength-sensitive and require less stringent fabrication tolerances. Moreover, they show very low attenuation and very low optical-signal reflection in the input channel which is important, particularly in the case of coherent detection employing narrow-band lasers. As the material indium phosphide is not birefringent, an implementation of a polarization splitter corresponding to that in lithium niobate or that in polable polymer is not possible. It is, however, possible, in thin light-guiding layers of non-birefringent materials, to implement waveguides which have different propagation behaviour for the two polarizations and in which, therefore, birefringence occurs. Birefringence of this type is known under the names waveguide birefringence, geometric birefringence or shape birefringence. It is caused by waveguides being formed on the surface of a substrate. The polarization in which the dominant electric field component is perpendicular to the surface of the substrate, in this case the TM polarization, as a result experiences a different propagation than the polarization parallel to that surface, in this case the TE polarization. This effect can be influenced either, as disclosed by reference [5], by arranging a suitable layer above or below the waveguide in a planar waveguiding layer, or, as disclosed by reference [6], by arranging a composite layer structure ("superlattice"), in which case, by specific choice of the layer structure, channel-shaped waveguides are obtained which are selective for one of the two polarization modes. Reference [7] discloses polarization-manipulating 3-gate and 4-gate devices, including a polarization splitter of the second kind, in which shape birefringence of this type, based on a suitably chosen layer structure, is applied. A 3-gate device such as a polarization splitter comprises two waveguides having cores of different materials and with different effective refractive indices. In a transition section, between a first waveguiding section and a second waveguiding section, the cores overlap and one of the waveguide cores has an adiabatic taper. In the transition section, the two waveguides diverge adiabatically into physically separate and optically decoupled waveguides in the second waveguiding section. Integrated optical devices, in which shape birefringence of this type, based on a suitably chosen layer structure is applied, have the drawback, however, that their manufacture is rather laborious, not only owing to the number of necessary manufacturing steps, but also owing to the number of different materials to be used.